138 Statics. 



* 



2, The more rough the surfaces, or the less they are planed 

 or polished, the greater is the friction. This resistance, there- 

 fore, may be diminished, by smoothing the surfaces, or by filling 

 the openings and pores with other matter, as oil, soap, grease y 

 black-lead, &c., with any substance indeed, which, while it fills 

 the cavities, does not give rise to a new adhesion. 



(3.) It would seem that the extent of surface ought to contrib- 

 ute sensibly to the friction; it appears, however, by a great va- 

 riety of experiments, that this circumstance makes but little 

 difference ; we find in fact the same difficulty for the most part 

 in drawing a body upon one of its surfaces as upon another, 

 though very different in extent, provided they are equally 

 smoothed. Thus, oak rubbing on oak, is found to have a fric- 

 tion of about 44 per cent; and on diminishing the surface as 

 much as possible, this is reduced only to 4H per cent. We 

 must except, however, the case of bodies resting upon a point, 

 when the friction is more considerable, than when the contact 

 takes place in several points. 



(4.) It is principally from pressure that friction arises, and 

 this resistance is found to increase in proportion to the pressure ; 

 that is, we require twice the force to overcome the friction when, 

 the weight is doubled, other things being the same. 



(5.) Still the time, during which the two surfaces are acting 

 upon each other, either by their own gravity, or by any other 

 force, is to be taken into the account, although its effect has not 

 yet. been accurately determined ; it is found that the augmentation 

 depending on this cause, has its limits, and that these limits vary 

 according to the nature of the rubbing surfaces. Coulomb found 

 that in wood sliding on wood, without grease, the friction at first 

 increased, but in a minute or two came to a limit, which it did 

 not afterward exceed. Oak, for example, sliding on oak, though 

 the pressure was varied from 74 lb * to 2474 lb - had a friction 

 after- a minute always nearly 44 parts in the hundred. 



In the case, however, of iron rubbing on iron, or iron on brass, 

 the friction is the same whether the bodies are just beginning to 

 move from rest, or have acquired any given velocity. 



